Conventionally, as one of the exhaust purification apparatuses that purify NOx in exhaust, an apparatus has been proposed in which a selective reduction catalyst is provided in an exhaust channel that selectively reduces NOx in the exhaust by adding a reducing agent. For example, with the selective reduction catalyst of a urea addition type using urea water as the reducing agent, ammonia is generated from the urea thus added, and NOx in the exhaust is selectively reduced by this ammonia.
With such a selective reduction catalyst, in a case of the injection amount of the reducing agent being less than an optimum amount, the NOx reduction rate declines from the ammonia being consumed in the reduction of NOx being insufficient, and in a case of being larger than this optimum amount, the ammonia that has become surplus in the reduction of NOx is discharged. As a result, appropriately controlling the injection amount of reducing agent has been important in exhaust purification apparatuses provided with a selective reduction catalyst. Therefore, in Patent Document 1 and Patent Document 2, apparatuses are exemplified that estimate a NOx reduction rate of a selective reduction catalyst, and control an injection amount of the reducing agent based on this estimation.
With the exhaust purification apparatus of Patent Document 1, the NOx concentration on a downstream side of the selective reduction catalyst is detected, and the composition of the exhaust flowing into the selective reduction catalyst, more specifically, the ratio of NO and NO2, is estimated from the NOx concentration thus detected and the operating state of the internal combustion engine. Furthermore, based on this composition of the exhaust, the NOx reduction rate of the selective reduction catalyst is estimated and the injection amount of reducing agent is controlled.
In addition, with the exhaust purification apparatus of Patent Document 2, the temperature of the catalyst is detected as an amount relating to the NOx reduction rate of the selective reduction catalyst, and the injection amount of reducing agent is controlled based on this temperature.
However, the NOx reduction rate of the selective reduction catalyst changes not only by the above such composition of the exhaust and temperature of the selective reduction catalyst, but also according to the degradation state of the selective reduction catalyst. In addition, there is variability in purification performance between individual units. In addition to this, in a case of ammonia having been stored in the selective reduction catalyst, the NOx reduction rate of the selective reduction catalyst changes upon appearance due to the optimum amount of reducing agent differing. Therefore, it is difficult to always optimally control the injection amount of reducing agent with exhaust purification apparatuses such as those exemplified in Patent Documents 1 and 2.
Consequently, a technique is considered below that more directly detects the NOx reduction rate of the selective reduction catalyst, and controls the injection amount of reducing agent based on this.
FIG. 29 is a schematic diagram showing a configuration of a conventional exhaust purification apparatus 80.
As shown in FIG. 29, an oxidation catalyst 83, urea injection valve 85 that injects urea water as a reducing agent that is stored in a urea tank 84 into an exhaust channel 82, and a selective reduction catalyst 86 that reduces NOx in the exhaust under the presence of urea water are provided in sequence from an upstream side to a downstream side in the exhaust channel 82 of an engine 81. In addition, a temperature sensor 87 that detects a temperature of the selective reduction catalyst 86 and a NOx sensor 88 that detects a NOx concentration on a downstream side of the selective reduction catalyst 86 are provided as sensors for observing the purification performance of the selective reduction catalyst.
With this exhaust purification apparatus 80, for example, the NOx concentration of exhaust discharged from the engine 81 is estimated by way of a map set in advance, and the injection amount of urea water by way of the urea injection valve 85 is determined based on this NOx concentration and the catalyst temperature detected by the temperature sensor 87. In particular, herein, degradation of the selective reduction catalyst 86 can be estimated based on a difference between the NOx concentration detected by the NOx sensor 88 and the NOx concentration of exhaust thus estimated. With this exhaust purification apparatus, it has been possible to correct the injection amount of urea water in accordance with the degradation state of the selective reduction catalyst 86 estimated in the above way.    Patent Document 1: Japanese Unexamined Patent Application Publication No. 2006-274986    Patent Document 2: Japanese Unexamined Patent Application Publication No. 2004-100700